Origins

In 1800, the physicist William Herschel placed a mercury thermometer in the spectrum produced by a glass prism, to measure the heat of the different bands of coloured light. He discovered that the thermometer continued to rise also after being moved beyond the red edge of the spectrum, where there was no more visible light. This was the first experiment that demonstrated how heat can be transmitted by an invisible light form.

infrared radiation

In physics, infrared radiation (IR) is electromagnetic radiation with a frequency lower than that of visible light, but greater than that of radio waves. The term means "below red" (from the Latin infra "below"), because red is the colour visible with the lowest frequency. The set of the infrared frequencies is called an infrared band  and is therefore a part of the entire electromagnetic spectrum.

Infrared radiation has a wave length (which is equal to the speed of light divided by the frequency) of between 700 mm and 1 mm. It is often association with the concepts of "heat" and "thermal radiation", because each object with a temperature above absolute zero (basically, any real object) spontaneously emits radiation in this band (according to Wien's law, by increasing the temperature, the emission peak continues to always move more towards the visible until the object becomes incandescent).

Infrared irradiators, overall, cover the range of wave lengths from 0.75 to 12 microns, and a filament temperature that ranges between 400 and 2500 degrees centigrade. The radiation penetrates more into the material the shorter the waves are. This consideration is valid, but the ability of the material to reflect the radiation must be taken into consideration.

• Technical sheet

The figure shows that there is a direct relationship between the temperature of the lamp filament and its wave length. As the temperature increases, the length of the emitted wave decreases. The position of the peak within the emission spectrum determines its type.

• view graph

*(text extracted from wikipedia)